1. Technical Field
The present invention relates generally to a pneumatic radial tire suitable for use on a passenger car, light truck, or the like. In particular, the present invention relates to an improved belt package structure of the tire.
2. Description of the Prior Art
It is known that the automotive industry continually seeks to improve vehicle fuel economy. In fact, many national governments throughout the world either have or are proposing mandated minimum fuel economy standards. In the United States of America, for example, minimum standards for corporate average fuel economy are constantly increasing. Thus, the automotive manufacturers must respond and seek ways to improve the minimum average fuel economy for the entire line of cars sold during a single model year.
To improve vehicle fuel economy, the automobile manufacturers work with tire manufacturers to decrease the rolling resistance and the overall weight of a tire used on a vehicle. Rolling resistance is defined herein as the amount of energy loss attributable only to the tire as the tire deforms at a contact patch during rotation. The tangential force exerted on the tire at the contact patch to make it roll divided by the load carried by the tire is the rolling resistance coefficient. The greater the energy loss the more the rolling resistance and the greater the work required for rotating the tire independent of moving the vehicle. This relatively greater work is required because of the various materials used in the tire and their properties at operating temperatures such as stress-strain magnitude and hysteresis.
One way that automobile manufacturers improved rolling resistance was to adopt the use of radial pneumatic tires in place of bias tires on their vehicles. It is known that the use of radial tires on a vehicle in place of bias tires leads to improvements in fuel economy. The tire 1 of FIG. 1 is typical of the known radial tires conforming to the 1992 Tire and Rim Association (T&RA) standards, herein called the "industry standards". A typical radial tire includes a pair of spaced apart and circumferentially inextensible beads. A carcass ply 7 having reinforcing members disposed in a generally radial direction extends between the beads and is attached at its axially opposite end portions to a respective one of the beads. The tire also includes a tread portion 2 and a pair of sidewall portions. The tread portion 2 is located in a crown region of the tire 1. A belt package 8 is disposed between the carcass ply 7 and tread portion 2. The belt package 8 of this prior art design includes two belt plies 4 and 5. The belt plies 4,5 each have equal spaced reinforcing members disposed at equal angles but in opposite directions with respect to the midcircumferential plane R. The tire 1 rotates about its axis of rotation A and has a midcircumferential plane R perpendicular to the axis of rotation A. A two-belt passenger tire 1 conforming to industry standards has a first crown thickness D1. The first crown thickness D1 comprises the thickness of the carcass ply 7, the thickness B1 of the two-belt package 8, the thickness S1 of the subtread 3 and the depth G1 of the groove of the tread 2.
The radial pneumatic tire 1 can be further designed to have construction features that reduce its rolling resistance. The crown area of the tire, comprising the tread and belt package, can be designed to limit deformation due to forces (stress vs. strain) that produce energy losses from rolling contact with the road surfaces. Rolling resistance is directly proportional to this energy loss. Deformations of the tire crown area are reduced by providing a belt package which has a higher rigidity of stiffness. Such a belt package will maintain its shape better when subjected to the same external forces. High inflation pressure in the tire also has a stiffening influence on the tire and is known to improve rolling resistance, but to the detriment of ride comfort and wear.
Ways of decreasing rolling resistance of a tire are often approached from optimizing the footprint or contact patch of the tire. Such improvements are intended to approach a theoretically ideal shape for the footprint or contact patch. It is generally felt that improvements to the footprint, or contact patch of the tire, can be achieved by stiffening the belt package of the tire. This may be accomplished by adding a third belt to the belt package. However, these improvements may adversely affect the mass of the tire and/or the ride comfort of the vehicle as well as the tread life of the tire.
Ways of stiffening the belt package in a tire are disclosed in U.S. Pat. Nos. 2,493,614, 3,945,422 and 4,526,217. The U.S. Pat. No. 2,493,614 discloses a tire with three belts. The first and second belts located closest to the carcass each have reinforcing members extending at an angle of 17 to 28 degrees with respect to a midcircumferential plane but in opposite directions. The third belt radially outward of the first and second belts has reinforcing members extending at an angle in the range of 45 to 90 degrees with respect to the midcircumferential plane. The reinforcing members in the belts form a series of triangles which produce a desired rigidity of the belt package. The order of superposition of the belts may be selected as desired to vary rigidity of the belt package by triangulation. Ride comfort, wear and endurance of the belt package are not optimized in a tire as disclosed in this patent.
The U.S. Pat. No. 3,945,422 discloses a radial pneumatic tire in which the belt package includes three annular belts. Each of the annular belts includes a plurality of parallel extending steel alloy reinforcing members. The radially innermost first belt has the reinforcing members disposed at an angle of 24 degrees relative to the midcircumferential plane of the tire and a width of 6 inches. The number of reinforcing members is 9 reinforcing members per inch taken in a direction perpendicular to the extent of the reinforcing members. A second belt disposed radially outward from the first belt has reinforcing members disposed at an angle of 29 degrees relative to the midcircumferential plane but opposite to the direction of the reinforcing members in the first belt. The second belt has a width of 4.8 inches and the number of reinforcing members is 18 per inch taken in a direction perpendicular to the extent of the reinforcing members. A third belt disposed radially outward of the second belt has reinforcing members disposed at an angle of 29 degrees relative to the midcircumferential plane in the same direction as the reinforcing members in the first belt. The third belt has a width of 5.4 inches and the number of reinforcing members is 9 per inch taken in a direction perpendicular to the extent of the reinforcing members. The relative width of the belts is wide, narrow and intermediate when progressing radially outward. All three belts have a different width and belt reinforcing members are at most 29 degrees relative to the midcircumferential plane.
The U.S. Pat. No. 4,526,217 discloses a tire having a belt package with three annular belts. Each of the belts include a plurality of parallel extending metal cords, such as steel. This three ply belt package also provides improved stiffening of the belt package of the tire. The angle of the reinforcing members in the first belt is between 25 degrees to 50 degrees relative to the midcircumferential plane of the tire. In comparison, the second and third belt reinforcing members extend at an angle of 10 to 30 degrees relative to the midcircumferential plane. The reinforcing members in the first belt and the second belt extend in the same direction relative to the midcircumferential plane of the tire. The direction the reinforcing members in the third belt extend is opposite to the direction reinforcing members in the first and second plies extend. The intended use of this tire is for heavy duty applications. However, the rigidity of the belt package is disclosed in this patent as a desirable feature for running on good roads at relatively high speed as well as on bad roads and non-paved roads of construction sites.